Holographic non-isotropic diffusing screen

ABSTRACT

A method and apparatus for constructing a holographic nonisotropic diffusing screen to produce a diffuse light beam of predetermined directionality having use in micro-holographic recording and reading apparatus.

La hula. 11s. 1 .114

United State: McMahon [54] HOLOGRAPHIC NON-ISOTROPIC DIFFUSING SCREEN[75] Inventor: Donald H. McMahon, Carlisle,

Mass.

[73] Assignee: Sperry Rand Corporation [22] Filed: April 28, 1971 [21]Appl. No.: 138,228

[52] US. Cl ..350/3.5, 161/3.5 [51] Int. Cl. ..G02b 27/22 [58] Field ofSearch ..350/3.5; l6l/3.5

[56] References Cited UNITED STATES PATENTS 3,523,054 8/1970 Heflingeret al ..350/3.5

ISOTROPIC HOLOGRAPHI gg RECORDING PLATE F Jan. 2, 1973 OTHERPUBLlflATlONS Brooks et al., QE-Z IEEE J. Quant. Elec." 275-279 (8/1966)Sincerbox, 10 IBM Tech. Disc. Bull. 267, 268 (8/1967) PrimaryExaminer-David Schonberg Assistant Examiner-Robert L. ShermanAttorney-S. C. Yeaton [57] ABSTRACT A method and apparatus forconstructing a holographic non-isotropic diffusing screen to produce adiffuse light beam of predetermined directionality having use inmicro-holographic recording and reading apparatus.

8 Claims, 4 Drawing Figures REFERENCE BEAM PA'IENTEDJAM' 2 1913 3.708217 SHEET 1T)? 2 MIRROR BEAM BEAMSPLITTER SIGNAL ISOTROPIC HOLOGRAPHIC ZSRECORDING PLATE HOLOGRAPH |c NON-ISOTROPIC ig TRANSPARENCY SIGNALHOLOGRAPHIC RECORDING PLATE REFERENCE BEAM F I/VVE/VTUR DONALD H MaMafia/v A T TOR/V5) PATENTEDJAM 2191s 3.708.217 SHEET 2 0F 2 I HHOLOGRAPHIC NON ISOTROP|C DIFFUSER HOLOGRAPHIC CONVERGING RECORDINGREFERENCE BEAM ISOTROPIC DIFFUSER FIG.4.

INVENTOR DONALD H. Ma MAHO/V ATTORNEY BACKGROUND OF THE INVENTION l.Field of the Invention The presentv invention relates to holography andmore particularly to a holographically constructed diffuser forproviding a highly directional diffuse light beam.

2. Description of the Prior Art Diffusing screens have generalapplicability in optical and holographic systems for scattering light,as required for example in certain non-coherent character recognitionsystems and micro-holographic recording and reading devices'A diffusingscreen is characterized by the property of uniformly scattering incidentlight so that light from every point on the screen is directed to everypoint in some other plane, an observation or recording plane. Thisproperty becomes evident when an image is projected onto one side of anisotropic diffuser constructed from opal or ground glass to be viewed byan observer on the other side of the screen. Isotropic or uniformscattering enables the observer to view the image from any position infront of the screen; in other words, the image is presented through awide field of view. This feature, while desirable in some applications,is undesirable in others, for instance where the image is required to beobserved by only a single viewer as in the case of a microholographicreadout device. A better understanding of the problem will be obtainedafter reading the subsequent description of the preferred embodiments;but for the moment suffice it to say that considerable, in fact most, ofthe available light energy is not used in a micro-holographic readerwhen the reconstructed image is presented through a uniform diffusingscreen. On the other hand, the diffusing screen cannot be eliminatedaltogether without the need for introducing an alternative element suchas a lens which has the disadvantage of materially tightening positionaltolerances of both the system components and the location of theobserver for undistorted viewing. Likewise, in a micro-holographicdiffuse recording system where it is desired to converge a diffusesignal beam, as opposed to the focused beam ofa Fourier transformrecording, onto a small region of a holographic plate in superposedrelation with a reference beam, it will be appreciated that considerablelight loss will result when the signal beam is passed through a uniformdiffuser prior to transmission through a data transparency containingthe information to be recorded. It is thus seen that undesired lightlosses occur in both recording and reading apparatus incorporatingconventional uniform diffusing screens. In view of this problem, thepresent invention is directed to a means and technique for constructinga directional diffusing screen which retains the property of aconventional diffuser, namely of scattering light from every point inthe diffusing plane to every point in some other plane, but in such away that the light in said other plane is concentrated in apredetermined area.

SUMMARY OF THE INVENTION Apparatus for constructing a non-isotropic(directional or non-uniform) diffuser screen in accordance with theteaching of the present invention comprises a light source for providingcoherently re- Iated, angularly separated reference and signal beamsdirected onto a recording'holographic plate in superposed relation. Thereference beam is preferably expanded by the combination of a shortfocal length lens and a collimating lens to produce a large diameterbeam impinging on the holographic plate. An isotropic diffuser screendisposed in the path of the relatively smaller diameter signal beamuniformly scatters the signal beam to produce a diffuse diverging beamwhich strikes the holographic plate to interfere therein with theexpanded reference beam and thereby produce a pattern representative ofthe size of the signal beam incident on the isotropic diffuser. Afterdeveloping in accordance with conventional techniques the holographicplate constitutes a hologram which functions as a directional diffuserby virtue of reconstructing the diffuse signal beam when illuminated byan appropriate reference beam. Stated somewhat differently, theimprovement of the present invention resides in the utilization of anisotropic diffuser alone in place of the object or input transparencynormally employed in holographic recording systems or, alternatively, inplace of the combination of an input transparency and an isotropicdiffuser in order to form a hologram of the diffuser capable ofproviding a diffuse beam of predetermined directionality whenilluminated by a reconstructing beam. A more detailed explanation of themethod and apparatus of the invention and the use thereof inmicro-holographic recording and reading systems is provided in thefollowing description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an optical schematicillustration of apparatus for constructing a non-isotropic diffusingscreen in accordance with the principles of the present invention.

FIG. 2 is a schematic illustration of micro-holographic recordingapparatus using a non-isotropic diffuser constructed with the apparatusof FIG. 1.

FIG. 3 is a schematic illustration of a micro-holographic readerapparatus incorporating a non-isotropic diffuser.

FIG. 4 is a schematic illustration of an alternate means forconstructing a non-isotropic diffusing screen for use in the readerapparatus of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, apparatusfor constructing a holographic non-isotropic diffuser comprises a laser10 emitting a light beam II directed toward beam splitter 12 whichdivides the beam into a reference beam 13 and a signal beam 14. Thereference beam reflects from mirror 15 onto microscope objective orother short focal length lens 16 which produces a widely diverging beamthat is collected and collimated by lens 17 for transmission toholographic recording plate 18. A reference beam diameter of about 15centimeters impinging on the holographic plate is suitable forconstructing non-isotropic diffusers for use in micro-holographicdevices. The signal beam simultaneously impinges on isotropic diffusingscreen 19 and is scattered therethrough in the form ofa diffuse beamdenoted by lines 20, 20' onto holographic plate 18 in superposed 3relation with the expanded reference beam to produce aninterferencepattern representative of the diffusely scattered signalbeam. It will be understood, of course, that the signal beam is actuallyscattered uniformly in all directions, that is, into a spheresurrounding the isotropic diffuser, so that only a fraction of thescattered light energy is actually confined within lines 20, andtherefore effective in producing the holographic pattern. Moreover, asindicated in the drawing, the size of the signal beam incident on theisotropic diffuser and the position of the latter relative to theholographic plate 18 determines the size of the solid angle confinedwithin lines 20, 20'. An apertured plate could be used on either side ofthe isotropic diffuser to control the signal beam size but this isgenerally not desired since it is wasteful of light energy.

Upon being developed, the holographic recording plate I8 constitutes aholographic non-isotropic diffuser which functions to provide a highlydirectional diffuse beam as will be understood more fully afterconsidering its use in the micro-holographic diffuse recording apparatusof FIG. 2 wherein the holographic recording plate 21 is an unexposedfilm or other holographic recording media similar to that used in theapparatus of FIG. I. Holographic plate 21 is positioned in back of iris22 to receive signal and reference beams 23 and 24, respectively,transmitted through the iris aperture. As in the case of the FIG. 1apparatus, the signal and reference beams are typically derived from acommon laser (not shown) so as to be coherently related. The referencebeam is directly incident on holographic plate 21 while the signal beampropagates successively through the holographic non-isotropic diffuser18' and transparency 26 containing the data to be recorded on theholographic plate.

It will be noted that signal beam 23 striking the nonisotropic diffuser18' is preferably of the same diameter as the reference beam 13 of FIG.1 at the point of incidence on holographic plate 18 and directed toimpinge on the non-isotropic diffuser from a direction opposite to theincidence of reference beam 13 on holographic plate 18. Under theseconditions, the signal beam 23 is diffracted by the non-isotropicdiffuser I8 to produce a directional diffuse beam confined within lines25, conforming to the relative angles of lines 20, 20 of FIG. 1. Thus,all or at least most of the light scattered by the non-isotropicdiffuser is directed through the aperture in iris 22 rather than beinguniformly radiated into a sphere as in the case of a uniform diffuser.

For a recording situation in which the iris 22 is the same distance fromnon-isotropic diffuser 18 as the isotropic diffuser 19 was displacedfrom the holographic recording plate 18 in the apparatus of FIG. 1, itwill be appreciated that the iris aperture should be of approximatelythe same size as that of the signal beam incident on the isotropicdiffuser for efficient use of the diffracted diffuse light provided bythe non-isotropic diffuser. The iris can be placed relatively closer orfurther away from the non-isotropic diffuser, if desired, but in eachinstance will have to be made relatively larger or smaller,respectively, for precise accommodation of the diffracted diffuse beam.

An appreciation of the degree of gain in optical brightness that accruesfrom the use of a non-isotropic diffuser as compared to the prior artisotropic diffusers will be obtained from the following quantitativeexample. Assume that a conventional opal or ground glass diffuser isused at a distance of 10 centimeters from an iris aperture having anarea of 0.05 square centimeters, a typical value for a micro-holographicdiffuse record ing system. With this setup the diffused light will bescattered uniformly into a sphere having a surface area of 1,200 squarecentimeters at 10 centimeters from the diffuser and, consequently, only0.05/l ,200 of the scattered light will pass through the iris apertureto the recorder holographic recording plate. Since only a very smallfraction of the available light is utilized, either unduly long exposuretimes or greatly increased laser power will be required to make theholographic recording. By the provision of a non-isotropic diffuser,however, which can be made to concentrate the light on an area of 1square centimeter or less, the brightness of the signal beam directedonto the recording plate is increased substantially thereby enablingsuitable recordings to be made with considerably shorter exposure timesand/or much lower laser power.

A micro-holographic reader apparatus for reading out the data recordedin the apparatus of FIG. 2 is shown in FIG. 3. The reader comprises alaser 30 directing a light beam 31 onto holographic storage plate 21(the recording plate of FIG. 2) to produce a diverging data beam 32which provides a reconstructed image of the holographically stored data(the transparency data of FIG. 2) at the plane of non-isotropic diffuser33. The non-isotropic diffuser, in turn, scatters the reconstructedimage light in a manner to produce the converging beam 34 preferentiallydirected to a small area on the order of 5 X 15 centimeters at theobserver location indicated by the eye symbol 36.

In view of the previous description relating to the apparatus of FIG. 1,it will be appreciated that a collimated reference beam is not suitablefor constructing the holographic non-isotropic diffuser if it is desiredto obtain a converging image beam 34 as shown in FIG. 3. Accordingly,apparatus for constructing a non-isotropic diffuser for use in thereader of FIG. 3 is shown in FIG. 4. Here again the signal beam 37 andconverging reference beam 38 are preferably derived from a common lasersource so as to be coherently related. An isotropic diffuser 39uniformly scatters the signal beam in the manner explained withreference to FIG. 1 so that scattered signal light confined within lines35, 35' impinges on holographic recording plate 41 in superposedrelation with the reference beam 38 which converges to point P. Afterdevelopment of the holographic recording plate 41. it constitutes anon-isotropic diffuser suitable for use in the apparatus of FIG. 3.

The similarity of the geometry of FIGS. 3 and 4 will be noted. In theapparatus of FIG. 4 a diverging beam denoted by lines 40, 40' willreconstruct a directional diffuse signal beam confined within lines 35,35'. Likewise, in the apparatus of FIG. 3, the small area of theholographic recording 42 (approximately 0.05 square centimeters) can beregarded as a point source providing a data beam 32 of the same angulardimension as the beam emanating from point P. Hence, when theholographic storage plate 21' is located at about the same distance fromnon-isotropic diffuser 33 as point P is removed from the recording plate41 of FIG. 4, the

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converging image beam 34 will have the same diameter. at a distance fromthe non-isotropic diffuser 33 equal to the distance between theisotropic diffuser 39- and recording plate 41. as that of the signalbeam emanating from isotropic diffuser 39. It will be ap preciated,however, that the relative distances used in the recording and readingapparatus can be different if so desired for one reason or another.

While the invention has been described in its preferred embodiment. itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes may be made withinthe purview of the appended claims without departing from the true scopeand spirit of the invention in its broader aspects.

1 claim:

1. A method for providing a directional diffuse light beam ofpredetermined convergence, comprising the steps of providing first andsecond coherently related light beams for incidence on a holographicrecording plate in angularly separated and at least partially superposedrelation, positioning an isotropic diffusing means in the path of thefirst beam for uniformly diffusing said first beam,

disposing a holographic recording medium to receive the second beam anda diverging portion of the first beam emanating from the isotropicdiffusing means to record an interference pattern representative of thesize of said diverging portion at the location ofthe isotropic diffusingmeans,

removing the isotropic diffusing means and the first and second beams,and

propagating a light beam onto the holographic recording mediumantidirectional to the second beam whereby a diffuse converging beam isproduced conforming to the solid angle dimensions of said divergingportion of the first beam incident on the holographic recording mediumduring recording ofthe interference pattern.

2. The method of claim I wherein the second beam incident on theholographic recording medium is substantially larger than said divergingportion of the first beam at the location of the isotropic diffusingmeans.

3. The method of claim 1 wherein the first and second beams are derivedfrom a comparatively small laser beam and further including the step ofdisposing beam expanding and collimating means in the path of the secondbeam for directing the second beam onto the holographic recording mediumas a collimated beam substantially larger than the first beam impingingon the isotropic diffusing means.

4. A method of constructing a non-isotropic diffusing screen forproviding a diffuse light beam of predetermined convergence comprisingthe steps of providing first and second coherently related light beams,

positioning a holographic recording means in the path of the secondbeam, and

positioning isotropic diffusing means in the path of the first beam,absent a recording object. for uniformly diffusing said first beam suchthat a diverging portion thereof is incident in angularly separated andat least partially superposed relation with the second beam on sat hoographic recording means to form an interference pattern thereinrepresentative of the size of the first beam striking the isotropicdiffusing means whereby subsequent illumination of said holographicrecording medium with a single light beam propagated opposite to thedirection of the second beam reconstructs said diverging portion of thefirst beam incident on the holographic recording means to produce adiffuse converging beam.

5. The method of claim 1 wherein the second beam is a converging beamconverging to a point in back of the holographic recording medium on theside opposite the side on which the first and second beams impinge.

6. Apparatus for constructing a non-isotropic diffusing screen forproviding a diffuse converging light beam comprising means for providingfirst and second coherently re lated light beams,

holographic recording means disposed in the path of the second beam,

isotropic diffusing means positioned in the path of the first beam, inthe absence of a recording object, for uniformly diffusing said firstbeam so that a portion thereof diverging from said isotropic diffusingmeans impinges on said holographic recording medium in angularlyseparated and at least partially superposed relation with the secondbeam to form an interference pattern in said holographic recording meansrepresentative of the size of the first beam striking said isotropicdiffusing means whereby subsequent illumination of said holographicrecording medium with a single light beam propagated opposite to thedirection of the second beam reconstructs said diverging portion of thefirst beam incident on said holographic recording means to produce thediffuse converging beam.

7. The apparatus of claim 6 including beam expander means disposed inthe path of the second beam for enlarging the size thereof so as to besubstantially equivalent in size to said diverging diffuse beam at thelocation of the recording medium.

8. Holographic recording apparatus comprising holographic recordingmeans,

means including a light source for providing first and second spacedcoherently related light beams directed in angularly separated and atleast partially superposed relation onto said recording means forproducing an interference pattern representative of an object disposedin the path of the first beam intermediate said light source and saidrecording means, and

holographic non-isotropic diffusing means disposed proximate the objectfor directing light scattered from the object onto said recording meansas a diffuse converging beam to impinge on a prescribed area of therecording means.

i i i i

1. A method for providing a directional diffuse light beam ofpredetermined convergence, comprising the steps of providing first andsecond coherently related light beams for incidence on a holographicrecording plate in angularly separated and at least partially superposedrelation, positioning an isotropic diffusing means in the path of thefirst beam for uniformly diffusing said first beam, disposing aholographic recording medium to receive the second beam and a divergingportion of the first beam emanating from the isotropic diffusing meansto record an interference pattern representative of the size of saiddiverging portion at the location of the isotropic diffusing means,removing the isotropic diffusing means and the first and second beams,and propagating a light beam onto the holographic recording mediumantidirectional to the second beam whereby a diffuse converging beam isproduced conforming to the solid angle dimensions of said divergingportion of The first beam incident on the holographic recording mediumduring recording of the interference pattern.
 2. The method of claim 1wherein the second beam incident on the holographic recording medium issubstantially larger than said diverging portion of the first beam atthe location of the isotropic diffusing means.
 3. The method of claim 1wherein the first and second beams are derived from a comparativelysmall laser beam and further including the step of disposing beamexpanding and collimating means in the path of the second beam fordirecting the second beam onto the holographic recording medium as acollimated beam substantially larger than the first beam impinging onthe isotropic diffusing means.
 4. A method of constructing anon-isotropic diffusing screen for providing a diffuse light beam ofpredetermined convergence comprising the steps of providing first andsecond coherently related light beams, positioning a holographicrecording means in the path of the second beam, and positioningisotropic diffusing means in the path of the first beam, absent arecording object, for uniformly diffusing said first beam such that adiverging portion thereof is incident in angularly separated and atleast partially superposed relation with the second beam on saidholographic recording means to form an interference pattern thereinrepresentative of the size of the first beam striking the isotropicdiffusing means whereby subsequent illumination of said holographicrecording medium with a single light beam propagated opposite to thedirection of the second beam reconstructs said diverging portion of thefirst beam incident on the holographic recording means to produce adiffuse converging beam.
 5. The method of claim 1 wherein the secondbeam is a converging beam converging to a point in back of theholographic recording medium on the side opposite the side on which thefirst and second beams impinge.
 6. Apparatus for constructing anon-isotropic diffusing screen for providing a diffuse converging lightbeam comprising means for providing first and second coherently relatedlight beams, holographic recording means disposed in the path of thesecond beam, isotropic diffusing means positioned in the path of thefirst beam, in the absence of a recording object, for uniformlydiffusing said first beam so that a portion thereof diverging from saidisotropic diffusing means impinges on said holographic recording mediumin angularly separated and at least partially superposed relation withthe second beam to form an interference pattern in said holographicrecording means representative of the size of the first beam strikingsaid isotropic diffusing means whereby subsequent illumination of saidholographic recording medium with a single light beam propagatedopposite to the direction of the second beam reconstructs said divergingportion of the first beam incident on said holographic recording meansto produce the diffuse converging beam.
 7. The apparatus of claim 6including beam expander means disposed in the path of the second beamfor enlarging the size thereof so as to be substantially equivalent insize to said diverging diffuse beam at the location of the recordingmedium.
 8. Holographic recording apparatus comprising holographicrecording means, means including a light source for providing first andsecond spaced coherently related light beams directed in angularlyseparated and at least partially superposed relation onto said recordingmeans for producing an interference pattern representative of an objectdisposed in the path of the first beam intermediate said light sourceand said recording means, and holographic non-isotropic diffusing meansdisposed proximate the object for directing light scattered from theobject onto said recording means as a diffuse converging beam to impingeon a prescribed area of the recording means.